#include <stdio.h>
#include <math.h>

int main() {
	double s(int n);
	double  t;
	int n;
	scanf("%d", &n);
	printf("%f", s(n));
}

double s(int n) {
	double s = 0, a = 0;
	int i;
	for (i = 1; i <= n; i++) {
		a = a + sqrt(i);
		s = s + a;
	}
	return s;
}
